The present invention relates to a combustion system for internal combustion engines which is capable of operating in the extreme lean air/fuel ratio range. The use of lean air/fuel ratios makes possible the reduction of carbon monoxide emissions to near zero levels. Moderately lean operation up to 18:1 air/fuel ratio is undesirable because of the very high emissions of oxides of nitrogen (NO.sub.x) in this range. Although hydrocarbon emissions respond favorably to lean operation, most open chamber Otto engines start misfiring at ratios above 18:1 with resultant increase in hydrocarbon emissions. Misfire, of course, can also destroy an engine. The problem, then, becomes one of extending the misfire limit well into the extremely lean air/fuel ratios above 21:1 or 22:1 to create a comfortable operational range at ratios satisfactory to low values of all three major pollutants.
Although many efforts have been made to extend the lean misfire limit for open chamber Otto engines into the extreme lean region around 22:1, few have ever succeeded except in single cylinder laboratory engines.
The present invention includes a novel combustion chamber and igniting means system which utilizes and enhances the typically large unidirectional turbulence of combustion chambers with a side "pocket", to achieve not only positive ignition of extremely lean mixtures but also very fast combustion heretofore unachievable with either lean mixtures or retarded spark. This system utilizes both the lean mixtures and the retarded ignition and yet achieves fast combustion. In general, combustion with this system can best be described as controlled constant pressure combustion. The compression ratio is maintained as high as possible in accordance with the teachings of the present invention. The ignition timing is relatively late, in some cases starting even after the piston has passed TDC (top dead center). In this fashion, no combustion-generated negative pressures are exerted on the piston, but the maximum cylinder pressures are reduced considerably; and the time of their occurrence (in degrees) is somewhat later than in a conventional combustion system.
Combustion as described above results in direct reductions in NO.sub.x and HC emissions; but, in order to achieve improved fuel consumption without exhaust temperature deterioration, combustion, once established, must progress very fast so that the combustion pressures are exerted on the piston on the first 45.degree. of crankshaft movement as the piston moves down during the expansion stroke.
The combustion system described herein achieves the above explained results as follows:
1. Positive ignition of the mixture, even under lean conditions, is achieved by firing the spark plug which is strategically located above the piston, resulting not only from the natural motion of the gases from the piston onto the side chamber but very specifically from the rather large squish area surrounding this plug on three sides. This could be called primary ignition, or ignition by the leading plug.
2. Once ignition is assured by the firing of the first spark plug, the cylinder pressure and temperature start to increase. The high degree of turbulence still present in the chamber is further increased in the direction away from the already fired spark plug and towards the second spark plug located near or within what could be called the main charge. Combustion from the first spark plug is, however, somewhat controlled by the relatively low ignitable mass which in essence surrounds this plug, as well as by the rather shallow depth of the chamber formed between the upper portion of the piston and the cylinder head at this point in time (near TDC).
3. Ignition of the main charge follows that of the primary charge (primary ignition) by a number of crankshaft degrees. This ignition of the main charge could be called main or secondary ignition. Since the secondary or trailing plug fires in a highly turbulent environment, under higher pressures and temperatures than those to which a single plug cylinder is normally exposed, it is able to ignite the main charge under conditions otherwise impossible were this second plug to be the only source of ignition. From the moment that this trailing plug fires and secondary combustion starts, it proceeds very fast, not only because of the already mentioned favorable environment but because of the relatively compact and deeper portion of the chamber in the side pocket. But yet, the maximum cylinder pressure and temperature are kept under control, both because of the large amount of excess air present being intimately mixed with the products of combustion due to the highly turbulent combustion and because the piston is already on its downward stroke. This is how the NO.sub.x is kept under control.
The inherently large transfer of compressed mixture and/or combustion products to and from the cylinder and the side pocket traditional with "L" head designs, for example, is greatly minimized since a large portion of the actual combustion occurs on top of the piston. This reduction in pumping losses also helps to reduce the fuel consumption. The pumping losses are also reduced by the lowered manifold vacuums at which this engine operates.
The system, in accordance with the present invention, provides a novel approach to improving the brake specific fuel consumption, while at the same time decreasing all three major gaseous pollutants, reducing noise, reducing mechanical and thermal stresses, providing for fast starts even under cold conditions and in general resulting in very smooth operation.
The present invention is thus directed to a system for controlling the combustion rate in an internal combustion engine by reason of the shape and volume of the combustion chamber at several critical points and by the increased turbulence created by the final squish as the piston nears top dead center (TDC). To provide the critical control of NO.sub.x emissions, timing as between the firing of the two spark plugs will be relatively late. For internal combustion engines not requiring an extreme degree of NO.sub.x control, the timing can be advanced somewhat to improve power and fuel consumption. A major and outstanding advantage of the system is its control over NO.sub.x and other emissions through proper combustion chamber design, the use of extremely lean air/fuel ratios and the proper control of the spark timing on either plug. Additional advantages are that fuel consumption at peak load is greatly reduced by more complete fuel utilization and there is increased engine mechanical efficiency resulting from reduced pumping losses and friction. For the same speed and shaft horsepower with the lean mixtures, lower manifold vacuums result. This reduces the pumping losses, increases the mechanical efficiency and lowers the fuel consumption. It should be noted that the reduced manifold vacuums operate to reduce oil consumption for the engine.